Objectives To estimate the nationwide clinical and economic burden of influenza across five seasons (2014/15–2018/19) in South Korea, stratified by age and comorbidity, using data from the Hospital-based Influenza Morbidity & Mortality (HIMM) surveillance network. Methods Data were collected from eight university hospitals, including adults (≥20 years) with laboratory-confirmed influenza. The catchment population was calculated using hospital and national statistical data. Economic costs were evaluated from a societal perspective. Results During the study period, the incidence per 100,000 persons of medically attended influenza occurrence ranged from 113.1 to 220.7, with hospitalization at 35.5 to 76.8, intensive care unit admission at 4.2 to 9.2, and deaths at 1.4 to 3.6 among the adult population. The total socioeconomic costs ranged from $156 to $316 million annually, primarily due to indirect costs associated with early death, peaking during the 2017/18 season, which had the largest outbreak scale, particularly among older adults. In contrast, the per capita average costs, ranging from $2,747 to $4,072, were highest in the 2018/19 season, followed by the 2015/16 season. For at-risk individuals, the per capita average socioeconomic costs were 1.5–2.3 times higher than that of the general adult population. Conclusions Influenza poses a substantial economic burden in South Korea. Additional strategies are needed to alleviate morbidity and mortality in older adults and to protect at-risk individuals who are not currently covered by the National Immunization Program.

The authors have declared no competing interest.

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The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Boards of Korea University Guro Hospital (2019GR0475), Korea University Ansan Hospital (2020AS0045), Catholic University St. Vincent’s Hospital (VC20RIDI0056), Inha University Hospital (2020-01-013), Hallym University Kangnam Sacred Hospital (2020-01-004), Wonju Christian Hospital (CR320008), Kyung-pook National University Hospital (KNUH 2020-03-042), and Chonnam National University Hospital (CNUH-2020-064). Written informed consent was waived because of the study’s retrospective nature.

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